Method and apparatus for solution mining

ABSTRACT

A system for solution mining of minerals with heated solvent includes a single-bore hole provided with a string of pipe having a central conduit, an intermediate annular conduit surrounding the central conduit and an outer annular conduit enclosed by a casing. Prior to production recovery of the mineral, both the outer annular conduit and the intermediate annular conduit may be used for undercutting the mineral cavity if desired by pumping water through both conduits while recovering through the central conduit. The change from the undercutting to the production stage may be accomplished without loss of well pressure by pumping an oil-water-clay composition, herein sometimes referred to as a gel, an oil-water-clay gel, or an oil gel having a density less than that of water into the intermediate conduit without capping its bottom orifice to thereby thermally insulate the central conduit from the outer conduit. In production, heated water is pumped down the outer annulus and the chilled solution is recovered from the central conduit.

United States Patent [72] Inventors Robert Lee James Primary Examiner-Ernest R. Purser Winnetka; Allorneys-Dawson Tilton. Fallon & Lungmus andCarl C. William Lawrence Donovan, Palatine, both Batz of II]. [21] Appl.No. 50,373 [22] Filed 29 1970 ABSTRACT: A system for solution mining ofminerals with [45] Patented Nov. 30, I971 heated solvent includes asingle-bore hole provided with a t 73 Assignee Armour & Company stringof pipe having a central conduit. an intermediate annu- Chicago, [IL larconduit surrounding the central conduit and an outer annular conduitenclosed by a casing. Prior to production recovery of the mineral, boththe outer annular conduit and [54] METHOD AND APPARATUS FOR SOLUTION theintermediate annular conduit may be used for un- MINING dercutting themineral cavity if desired by pumping water 12 Claims I Drawing Figthrough both conduits while recovering through the central conduit, Thechange from the undercutting to the production [52] U.S.Cl 299/5 Stagemay be accomplished wihout loss f we" pressure by [51] Int. Cl E2lb43/28 pumping an oipwatepday compositiom herein sometimes [50] Fleld ofSearch 299/4-6 referred m as a gel an oiLwateFClay gel, or a n gelhaving a Reennces Cited density less than that of water into theintermediate conduit without capping its bottom orifice to therebythermally insu- UNITED STATES PATENTS late the central conduit from theouter conduit. In production. 2,l6l,800 6/1939 Cross 299/5 heated wateris pumped down the outer annulus and the 3,42 l ,794 l/I969 .lacoby299/5 chilled solution is recovered from the central conduit.

4 our W P R E S S U R E W34 V A L V E /3| AUXILIARY l P U M P PATENTH]NIJII30 I971 PRESSURE AUXILIARY mv/m' 1 ms: ROBERT L. JAMES WILLIAM L,DONOVAN B a/fi/fifia fi ATT'Y METHOD AND APPARATUS FOR SOLUTION MININGSUMMARY OF THE INVENTION In mining a potash cavity, a string of pipe isprovided including a central conduit, an intermediate annular conduitabout the central conduit and an outer annular conduit, allcommunicating with the potash and the underlying salt beds. The potashbed is undercut by pumping water through two of the conduits to dissolvethe underlying salt, and the solution is recovered from the thirdconduit which may, for example, be the central conduit. A back pressureis maintained on the discharged salt solution at a level sufficient togenerate a pressure in the cavity slightly less than the formationpressure When it is desired to change to the dissolving and productionof potash, an insulating oil-water-clay gel is pumped to fill theintermediate annulus without capping the down-hole aperture thereof andwhile maintaining the system pressure. During production, hot water ispumped through the outer annulus and the cooler solution is recoveredthrough the central conduit. By providing an oil-base gel in theintermediate annular conduit we have found that we provide a veryefiective insulating barrier between the inner and outer conduits; andthis minimizes the transfer of heat from the downcoming hot water andthe upcoming solution thereby allowing the introduction of heat into thecavity to replace most of the heat lost by the dissolving of the potashore.

It has been found that the intermediate annulus filled with anoil-water-clay gel having a thickness of about 1 inch provides anextremely good insulation so that a minimum amount of the heat in thehot water is lost to the solution being recovered. The oil-water-claygel remains in the intermediate annulus even though the down-holeaperture is not capped because the gel is lighter than water.

The invention, therefore, permits solution mining of potash with asingle well-bore while allowing the advantage of having three separateconduits communicating with the mineral cavity which may be usedindependently, if desired. It has been found that there is very little,if any, deterioration of the insulating properties of the oil-water-claygel over prolonged periods of use. There is no appreciable settling ofparticles from the gel even over extended periods of time, during whichpressure may, of course be maintained on the well. Further it has beenfound that there is no substantial displacement of the gel by the wateror brine even though the down-hole aperture of the intermediate annulusis not capped. Because the intermediate annulus is not capped, theplacement and removal of the drilling casing are greatly facilitated.

The unit insulating properties of the gel against heat conduction havebeen found to be approximately equal to a microscopically thin oil filmwhich is known to have excellent insulation effect. The gel minimizesconvection currents which normally take place in oil films having athickness greater than microscopic, and it thereby advantageouslyextends the excellent insulating properties of the oil into thicknessranges in which it would otherwise be ineffective.

Other features and advantages of the present invention will be apparentto persons skilled in the art from the following detailed description ofa preferred embodiment accompanied by the attached drawing.

THE DRAWING The drawing is a vertical view, in cross section, of asolution mining system illustrating the invention.

DETAlLED DESCRIPTION Turning then to the drawing, reference numeralgenerally designates pipe means including first inner or central tubing11, an intermediate tube or pipe 12 and an outer casing 13 which mayhave an exterior coating of cement if desired. The pipe 11 defines acentral bore 14. The space between the outer surface of the pipe 11 andthe inner surface of the pipe 12 forms an intermediate annular conduit15. The spacing between the outer surface of the tube 12 and the innersurface of the casing 13 forms an outer annular conduit 16. The pipemeans extends through the earth 18 into a subterranean layer of potash19 or other mineral to be recovered by solution mining. Typically, alayer of sodium chloride designated by reference numeral 20 existsbeneath the potash 19. The potash layer may have a thickness typicallyin the range of 20-40 feet. The overall depth of the pipe string may be4,0005,000 feet or more, and the down-hole aperture 23 of the centralconduit 14 is located at a depth of about l0 feet beneath the down-holeaperture 25 and 26 of the intermediate annular conduit 15 and outerannular conduit 16.

Prior to the production stage of recovering the potash, it may bedesirable to undercut the potash layer 19 as at 27, and because theintermediate annulus 25 is not capped, water for undercutting the sodiumchloride may be pumped both from a main hot water pump 30 connected viapipe 32 to the outer casing 13 for forcing water through the annularconduit 16 and from an auxiliary pump 31 connected to the intermediatetube 12 for supplying water to the annular conduit 15. A pressure valve34 is interposed in the central conduit 11 to couple the brine to anoutlet conduit 33 while maintaining system pressure. Persons skilled inthe art will appreciate that it is desirable to continue to maintainsystem pressure at all times once it has been achieved. The water usedfor undercutting the potash need not be heated to any significant degreein this case because the heat loss from dissolving the sodium chlorideis negligible and, .therefore, the temperatures between the ingoingwater and outgoing brine solution are about the same.

However, during the production stage hot water is pumped from the mainpump 30 through'the outer conduit 16, and a chilled solution containingthe dissolved potash is recovered through the central conduit 14 via thepressure valve 34. In this case, there is a considerable difference intemperature between the ingoing and outgoing fluids; and it is, ofcourse, highly desirable to maintain this temperature difference so thatthe hot water can be used to replace most of the heat lost due to thechilling effect of the dissolving of the potash.

Hence, during production, we use the auxiliary pump 31 to pump anoil-water-clay gel into the intermediate annular conduit 15 tosubstantially fill that conduit and thereby thermally insulate thecentral bore 14 against heat transfer from the outer annular conduit 16.

The insulating material in the annular conduit 15 is preferably lighterthan water. This has the advantage that the annular opening 25 need notbe capped thereby permitting the changing from the undercutting to theproduction stage without having to reduce system pressure. Secondly, byhaving the insulating material being a pumpable fluid which is lighterthan water and which does not mix with water, the insulating materialwill not be displaced from the annular insulating conduit 15 duringproduction but may be removed when the mining is terminated. Theinsulating properties of the system will not deteriorate appreciablywith time. However, prior attempts to use a fluid such as ordinarypetroleum oils in the insulating annular conduit 15 have provedunsuccessful because of the existence of heat convection currents inthat fluid. As already mentioned, the depth to which the pipe meansextends may be of the order of 4,0005,000 feet or even greater beneaththe surface of the earth.

Because the well is this deep and the mining becomes more expensive asbore size is increased, it is advantageous to utilize insulatingmaterials that are effective in thin layers. We have discovered that wecan attain these advantages and achieve yet further advantageousoperation by utilizing gels. Persons skilled in the art will appreciatethat such gels can be readily removed and replaced without affecting thepressure on the system.

EXAMPLE I In a preferred embodiment, we mix diesel oil with 2 percentwater by volume to which we add a gelling agent containing clay and soldas Geltone A and manufactured by the Baroid Division of National LeadCompany. It will be appreciated that many other petroleum oils may beemployed in place of the diesel oil with like results. The agent isadded at the rate of about 12.5 pounds of gelling agent per 42 gallonsofdiesel oil. We thus achieve a stable oil-water-clay composition which aslow enough viscosity so that it may easily be pumped into place by theauxiliary pump 31, and yet high enough viscosity to almost completelyeliminate convection currents in the insulating fluid. However, a rangeof about 8 to about 15 pounds of the gelling agent per 42 gallons (i.e.barrel) of oil with about 2-5 percent of water is believed to beeffective.

With the oil-water-clay gel sheath having a thickness (that isdifference between the outer radius and inner radius) of 0.933 inch, wehave been able to achieve an insulation factor, k, of 1.02 B.t.u. perhr. per foot per inch. This compares with a k of 1.002 B.t.u. per hr.per foot per inch for a microscopically thin film of diesel oil; and itis well known that such thin oil films have excellent insulatingqualities. Thus the invention achieves the advantage of having twoseparate inlet conduits, if desired, as well as an outlet conduit duringundercutting while permitting heat insulation of the centermost conduitfrom the outermost annular conduit in an annular piping means.

Further, the oil gel has a lesser density than water and/or salt andpotash brines and does not mix appreciably with them; and we have foundthat the gel will stay in place despite not having to cap the down-holeaperture 25 for long periods of time with no breakdown of the gel andwithout appreciable reduction in the insulating properties of the gel.With this system we are also able to easily recover the tubing andcasing which are reusable.

The filling of the annular conduit 15 with the oil gel is accomplishedby pumping the oil gel and measuring the volume to displace whateverfluid may be in that conduit.

With the oil gel as described above in the annular conduit 15 and thatconduit having a thickness of 0.933 inch, hot water at a temperature of160 and the depth of the well being about 4,400 feet, we found that thetemperature of the water exiting from the annulus 26 was about 150 andthat the temperature of the brine into the down-hole central aperture 23was about 122. The temperature of the fluid recovered form the pressurevalve 30 was about 117 F., showing little heat transfer between theliquids separated by the insulating gel.

Although we have achieved satisfactory results with the above gel, anyorganophilic clay such as montmorillonite or attapulgite may be usedwith various petroleum oils such as diesel oil, naphtha, and otherdistillate oils, and water provided the resultant gel is easily pumpableyet has high enough viscosity to eliminate or substantially reduce heatconvection currents in the intermediate conduit.

EXAMPLE 11 A gelling reagent may be prepared in the field by exchangingsuitable amines or amino salts in the presence of concentrated HCl on tosodium bentonite and using CMC (carboxylomethylcellulose) to bridge themolecules. Sodium bentonite is well known as a class of montmorilloniteclay which may be reacted by cation exchange to form the gelling agentas follows:

Na bentonite +C H NH HC1- C,,,H NH bentonite Further thickening may beachieved by adding a small amount of CMC which bridges between thebentonite/amine molecules. Weight ratios of the amine, sodium bentoniteand HCl are of the order l:l:3. Diesel oil with 2-5 percent water byvolume is blended into the system after the above reaction until a gelhaving the desired viscosity and pumpability are achieved (for exampleabout 12 pounds of the gelling agent per barrel of the oil and water).

With the inventive system we achieve the further advantage of having asingle-hole mining system which is capable of both undercutting, i.e.dissolving underneath, the potash layer, if

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desired, and readily converting to the production stage without reducingsystem pressure. It will be appreciated that two-hole systems areparticularly to be avoided where the beds of potash are thin and theydip appreciably.

Further, the invention represents a relatively inexpensive method ofachieving a very good insulation by using materials which are readilyavailable, easy to prepare and easy to install.

Further, because there is no bottom plug or cap to the intermediateannular conduit, the oil-water-clay gel insulating layer is part of theentire hydraulic system and can easily be removed by pumping displacingliquid down the outer conduit while closing the pressure valve 30 andreplacing the auxiliary pump 31 with a similar pressure relief outletvalve.

Another advantage of the system is that since the oil gel is part of theclosed fluid system, it may be used to record the fluid pressure in thesystem if this is more convenient than recording in either of the othertwo conduits.

Having thus described in detail a preferred method of solution miningusing a single drill-hole together with apparatus and material foraccomplishing the same, persons skilled in the art will be able tomodify certain of the steps described and to substitute materials orelements for those which have been disclosed or illustrated; and it is,therefore, intended that all such substitutions and modifications becovered as they are embraced within the spirit and scope of the appendedclaims.

We claim:

1. In a system for solution mining, the combination comprising pipingmeans including a first tube defining a central conduit, a second tubeabout said first tube and defining therewith an intermediate annularconduit, and an outer casing about said second tube and definingtherewith an outer annular conduit, said central conduit and saidintermediate and outer annular conduits communicating directly with adeposit of the mineral to be mined; means for pumping a heated fluidinto one of said central conduit and said outer annular conduit; astable gelled oil-water-clay composition having a density less than thedensity of water and of pumpable viscosity in said intermediate conduitto fill the same and thermally insulate said central and outer annularconduits from each other.

2. The system of claim 1 wherein said composition comprises sodiumbentonite mixed with a gelling agent and blended with a mixture ofpetroleum oil and water, said water being in the range of 2-5 percent ofthe mixture by volume and said sodium bentonite being in the range of8-15 pounds per barrel of oil.

3. The system of claim 1 wherein the lateral spacing between the innerand outer walls of said intermediate annular conduit is about 1 inch andthe thermal conductivity, k, of said composition in use is about 1.02B.t.u. per hour per square foot per inch ofgel.

4, The system of claim 1 further comprising pump means for selectivelypumping said composition into said intermediate conduit to fill thesame; and pressure valve means connected to the other of said centralbore and outer annular conduit for releasing brine therefrom whilemaintaining system back pressure.

5. The system of claim 1 characterized in that the downhole aperture ofsaid intermediate annular conduit remains open.

6. In a method of solution mining the steps of placing a pipe string ina well-bore, said string having a central bore, an intermediate annularconduit and an outer annular conduit, all communicating with the cavityof the mineral to be mined, pumping a heated solvent through one of saidcentral bore and outer annulus, recovering the resultant solutionthrough the other of said central bore and outer annulus, andmaintaining an insulating oil-water-clay gel in said inner annulus.

7. The method of claim 6 wherein said gel comprises sodium bentonitemixed with a gelling agent and blended with a mixture of oil and water,said water being in the range of 2-5 percent of said mixture by volumeand said sodium bentonite being in the range of 8-15 pounds per barrelof oil.

8. The method of claim 6 wherein said intermediate annular conduitdefines a spacing of about 1 inch between its inner and outer walls.

9. A method of solution mining of a mineral in a cavity located adjacenta body of a salt beneath the surface of the earth comprising installingconduit means in a well-bore communicating with the cavity including acentral conduit, an intermediate annular conduit about said centralconduit and an outer annular conduit about said intermediate conduit,all communicating said mineral with a surface location, forcing solventthrough two of said conduits while recovering dissolved salt from theother conduit to undercut said mineral cavity, then pumping a stableoil-water-clay gel in said intermediate conduit to thermally insulatesaid central bore from said outer conduit, then pumping a heated solventinto one of said central and outer conduits to dissolve said mineral,and

recovering the resultant solution with dissolved mineral from the otherof said conduits.

10. The method of claim 9 wherein said mineral is potash and said gel isan oil-water-clay composition mixed with a gelling agent to provide apumpable gel which minimizes transfer of heat by conduction between saidcentral and outer conduits.

l l. The method of claim 9 wherein said solvent is hot water introducedinto a potash cavity via said outer annular conduit and wherein thebrine is recovered from said cavity through said central conduit,

12. The method of claim 9 further comprising the step of sealing therecovery conduit and forcing a liquid into the entrance conduit to forcesaid gel out of said intermediate conduit, then recovering the drilltubing.

2. The system of claim 1 wherein said composition comprises sodiumbentonite mixed with a gelling agent and blended with a mixture ofpetroleum oil and water, said water being in the range of 2-5 percent ofthe mixture by volume and said sodium bentonite being in the range of8-15 pounds per barrel of oil.
 3. The system of claim 1 wherein thelateral spacing between the inner and outer walls of said intermediateannular conduit is about 1 inch and the thermal conductivity, k, of saidcomposition in use is about 1.02 B.t.u. per hour per square foot perinch of gel.
 4. The system of claim 1 further comprising pump means forselectively pumping said composition into said intermediate conduit tofill the same; and pressure valve means connected to the other of saidcentral bore and outer annular conduit for releasing brine therefromwhile maintaining system back pressure.
 5. The system of claim 1characterized in that the down-hole aperture of said intermediateannular conduit remains open.
 6. In a method of solution mining thesteps of placing a pipe string in a well-bore, said string having acentral bore, an intermediate annular conduit and an outer annularconduit, all communicating with the cavity of the mineral to be mined,pumping a heated solvent through one of said central bore and outerannulus, recovering the resultant solution through the other of saidcentral bore and outer annulus, and maintaining an insulatingoil-water-clay gel in said inner annulus.
 7. The method of claim 6wherein said gel comprises sodium bentonite mixed with a gelling agentand blended with a mixture of oil and water, said water being in therange of 2-5 percent of said mixture by volume and said sodium bentonitebeing in the range of 8-15 pounds per barrel of oil.
 8. The method ofclaim 6 wherein said intermediate annular conduit defines a spacing ofabout 1 inch between its inner and outer walls.
 9. A method of solutionmining of a mineral in a cavity located adjacent a body of a saltbeneath the surface of the earth comprising installing conduit means ina well-bore communicating with the cavity including a central conduit,an intermediate annular conduit about said central conduit and an outerannular conduit about said intermediate conduit, all communicating saidmineral with a surface location, forcing solvent through two of saidconduits while recovering dissolved salt from the other conduit toundercut said mineral cavity, then pumping a stable oil-water-clay gelin said intermediate conduit to thermally insulate said central borefrom said outer conduit, then pumping a heated solvent into one of saidcentral and outer conduits to dissolve said mineral, and recovering theresultant solution with dissolved mineral from the other of saidconduits.
 10. The method of claim 9 wherein said mineral is potash andsaid gel is an oil-water-clay composition mixed with a gelling agent toprovide a pumpable gel which minimizes transfer of heat by conductionbetween said central and outer conduits.
 11. The method of claim 9wherein said solvent is hot water introduced into a potash cavity viasaid outer annular conduit and wherein the brine is recovered from saidcavity through said central conduit.
 12. The method of claim 9 furthercomprising the step of sealing the recovery conduit and forcing a liquidinto the entrance conduit to force said gel out of said intermediateconduit, then recovering the drill tubing.